1. Field of the Invention
This invention concerns a procedure for the evaluation of the quality and/or the efficiency of a cable in a cable installation by a current measurement of a leakage current as a parasitic cross current.
2. Discussion of Related Art
It is known, that the parasitic cross current flows as a leakage current between a current carrying conductor at the core of a cable and the cable screen or concentric, which is generally an earth-ground potential. This current reduces the efficiency and/or the Quality of the transmission. By the measurement of this leakage current, the quality of the cable can be determined, additionally a corresponding test criteria can be established. The higher the leakage current, the lower is the quality of the cable.
For the evaluation of the leakage current, it was recommended to connect a DC voltage between the core and the screen of the cable and to measure the resulting flow of the current. For this purpose the cable is disconnected from the net and tested with a DC voltage equivalent to the operating voltage level. The leakage current is measured via a connected current meter. The disadvantage of this procedure is the generation of polarisation effects in the dielectric of the cable insulation resulting in a possible pre-damaging of the cable.
Further known methods are the so called VLF (very low frequency) Test methods. For this method, an alternating voltage with a very low frequency, for example 0.1 Hz is applied. The rising/falling slope of this frequency is in the frequency spectrum of 50 Hz. Significant disadvantages of this technology are interferences of compensation losses and leakage current which prevent a stable and continuous measurement of the leakage current.
Further known technologies as resonance testing have the same disadvantage, that the continuous measurement of the leakage current is not possible. This is a result of continuous re-charging procedures that prevent a stable situation in the cable.